LEDs are conventionally known as signalling devices, for instance an indicator that an apparatus is stand-by. With the development of high-power LEDs, LEDs are nowadays also used for illumination applications. In a particular application, a LED array is used in a tail light unit of a vehicle, specifically a motor vehicle. In such application, the LED array can have two different operational modes. In a first mode, the LEDs generate maximum light intensity. This is achieved by operating the LEDs continuously with nominal current (DC). In this mode, the LED array functions as brake light, so this mode is also referred to as “brake mode”. This brake mode is only active for as long as the vehicle driver actuates the brake pedal or brake handle.
In a second mode, the LEDs generate reduced light intensity. In this mode, the LED array functions as tail light, so this mode is also referred to as “tail mode”. Reducing the light intensity can be done by reducing the LED current. However, this is not preferred, because the color and efficiency of the LEDs may change if the current intensity changes. Therefore, reducing the light intensity is typically done by driving the LEDs with pulsed current, i.e. the current is alternatively zero or maximal (i.e. nominal). A time interval where the current is zero will be indicated as an OFF interval or pulse pause: during such interval, the LED is OFF. The duration of this interval will be indicated as OFF duration or pulse pause duration. A time interval where the current is nominal will be indicated as an ON interval or pulse: during such interval, the LED is ON. The duration of this interval will be indicated as ON duration or pulse duration. The average light intensity depends on the duty cycle, i.e. the ratio of the pulse duration (ON) to the duration of the pulse period (ON+OFF). A lamp driver is designed to keep the average light intensity constant by keeping the average current constant.
Driver circuits for driving an arrangement of LEDs with substantially constant current are already known. Typically, such constant current driver circuit comprises a current sensor for sensing the LED current, and a sensor signal is fed back to a controller, which controls a power source such that the sensed current is kept substantially constant at a predetermined level. The drive circuit further comprises a controllable switch connected in series with the lamp. In tail mode, the controller switches the switch OPEN and CLOSED alternatively. During the pulse intervals, the LED current is kept substantially at the nominal level; if current deviations occur, the influence on the average current is compensated by varying the duty cycle.
A problem may occur if the power source is a relatively slow power source. This problem typically may occur in case the power source is implemented as a converter, but the problem may also arise in the case of other types of power sources. In this respect, “slow” means that the time it takes for the power source to have its output current rise from zero to the nominal current level is longer than the pulse duration. During the pulse pause, the output current of the power source reduces, perhaps even drops back to zero, and at the start of the next pulse the current starts rising again. Thus, if the nominal level is reached at all, it may take many current periods until the LED current finally reaches this level, which means that the intensity of the tail lights reaches its operative level only very slowly.
The present invention aims to provide a drive circuit where this problem is overcome or at least reduced. More particularly, the present invention aims to provide a drive circuit which is capable of driving LEDs with pulsed current and which has improved start-up characteristics.